Plaster of Paris casts have been in use to immobilize body members or limbs for some time. In recent years, the plaster of Paris bandages have been superseded by synthetic casting tapes which employ polymeric materials on a fiberglass substrate. The polymeric materials are of the type that cure by exposure to actinic radiation or cure when reacted with water. More recently, water-cured or water-reactive polyurethane compositions have been used in forming orthopedic casts and the polyurethane compositions have largely supplanted other polymeric synthetic casting materials. The polyurethane casting materials are of the type which are disclosed in U.S. Pat. Nos. 4,667,661; 4,609,578; 4,411,262; and 4,376,438.
High modulus fibers such as fiberglass are commonly used as resin reinforcements in composite materials to impart strength to the cured article. In some applications the fiberglass yarn is woven or knit into a fabric which can then be treated with a finish, resin or other coating. Compared to organic fibers such as polyester, glass fibers exhibit virtually no elongation and are more prone to breakage during process operations such as knitting and weaving. Because the fiber elongation is essentially nil, glass fabrics do not stretch unless they are constructed with very loose loops which can deform upon application of tension, thereby providing stretching of the fabric. Knitting with loosely formed chain stitches imparts extensibility by virtue of its system of interlocking knots and loose loops.
An important usage of knitted fiberglass fabrics is in the manufacture of orthopedic cast bandages where those fabrics are coated or impregnated with a curable resin and packaged as rolls of casting tape, which are subsequently used by a physician or medical clinician to construct orthopedic casts. When constructing an orthopedic cast from a roll of casting tape (typically .about.100 millimeters (mm) wide), it is necessary to change the direction of wrapping to accommodate body contours such as the heel of the foot. This requires a high level of skill on the part of the applier to achieve a functional and smooth cast. To facilitate application, it is desirable that the bandaging material be extensible. At least 20% and preferably 25%-35% extensibility is necessary in material of 100 mm width to conform easily around the heel of a medium size adult.
Like most knitted fabrics, fiberglass knits tend to curl or fray at a cut edge as the yarns are severed and adjacent loops unravel. Fraying and raveling produce unsightly ends and, in the case of an orthopedic cast, frayed ends may interfere with the formation of a smooth cast, and loose, frayed ends may be sharp and irritating after the resin thereon has cured. Accordingly, frayed edges are considered a distinct disadvantage in orthopedic casting tapes.
Stretchy fiberglass fabrics which resist fraying are disclosed in U.S. Pat. No. 4,609,578 (Reed), the disclosure of which is incorporated by reference. Thus, it is well known that fraying of fiberglass knits at cut edges can be reduced by passing the fabric through a heat cycle which sets the yarns giving them new three-dimensional configurations based on their positions in the knit. Fiberglass fabrics intended to be coated or impregnated with a resin are sometimes put through a heat-setting process for the additional purpose of removing sizing from glass filaments which may otherwise interfere with the proper application of the resin.
A batch process recommended by Owens-Corning Fiberglass Corporation, Toledo, Ohio to remove the sizing on the glass filaments results in a heat-set fabric and consists of the following steps:
1. Start at 100.degree. C. for 1.5 hours; PA1 2. Raise to 250.degree. C. over a period of 5 hours and hold for 12 hours; PA1 3. Raise to 370.degree. C. over a period of 3 hours and hold for 33 hours; PA1 4. Cool to room temperature.
A continuous process involving much less time is also possible.
When a fiberglass fabric which has been heat-set is cut, there is minimal fraying and when a segment of yarn is removed from the heat-set fabric and allowed to relax, it curls into the crimped shape in which it was held in the knit. Accordingly, at the site of a cut, the severed yarns have a tendency to remain in their looped or knotted configuration rather than to spring loose and cause fraying.
In processing extensible fiberglass fabrics according to U.S. Pat. No. 4,609,578 (Reed), a length of fabric is heat-set with essentially no tension. The fabric is often wound onto a cylindrical core so large batches can be processed at one time in a single oven. Care must be taken to avoid applying undue tension to the fabric during wind-up on the knitter which would distort the knots and loops. To prevent applying tension to the fabric during winding, the winding operation is preferably performed with a sag in the fabric as it is wound on the core.
Prior to the present invention, rolls of moving webs of fabric were typically either (1) wound with some degree of tension (thereby distorting the knots and loops) or (2) wound loosely in the form of a wide web (e.g., &gt;50 mm width) so that the roll would not telescope and the web could be steered and uniformly processed. Unfortunately, narrow webs (i.e., webs having a width less than about 50 mm) easily telescope or collapse when wound loosely on the core and also easily "neck-down" (i.e., the narrow web distorts forming a narrower than desired width) in response to even minimal knitting or winding forces which are transmitted to the free edges of the tape. Wide webs are also susceptible to neck-down. These problems have made it difficult to and greatly increased the cost of producing narrow webs with good extensibility and webs of uniform width.